A wide variety of heating refrigeration and air conditioning systems are known which employ an evaporator, a condenser, an expansion valve or capillary tube, and a compressor. In such systems, low pressure refrigerant is compressed by the compressor and leaves the compressor as a vapor at an elevated pressure, and then condenses in the condenser, resulting in a transfer of heat to the environment surrounding the condenser. High pressure liquid then passes through an expansion valve in which some of the liquid refrigerant flashes into vapor. The remaining fluid is vaporized in the low pressure evaporator, resulting in a transfer of heat to the evaporating refrigerant from the environment. The refrigerant vapor is then drawn into the compressor, and the cycle begins again.
In some applications, the refrigerant may be cooled in the evaporator to a temperature which results in the formation of ice on the external surfaces of the evaporator. For example, the condenser of a heat pump typically forms an indoor coil of a system, and the evaporator forms an outdoor coil which extracts heat from the ambient air. During the heating cycle, ice may build up on the outdoor coil as water condenses on the coil because the temperature of the refrigerant in this coil is substantially below the freezing point of water. Accumulated ice may act as an insulator and provide a thermal barrier which interferes with heat transfer between the refrigerant in the evaporator and the outside environment. This in turn results in a significant decrease in the efficiency of the heat pump.
In order to avoid or at least inhibit this decrease in efficiency, procedures have been proposed to defrost the outdoor coils of heat pumps at regular intervals. Defrosting is typically performed by one of two procedures, both of which require the expenditure of substantial amounts of energy.
According to the first procedure, a resistive heating element is connected to the evaporator and is activated and deactivated as required to effect the defrost operation. While such external heat sources effectively defrost the evaporator, they are complicated construct, install, and control. In addition, they tend to be very energy intensive and in turn would decrease the efficiency of the heat pump.
The second common procedure for defrosting the evaporator of a heat pump involves the reversal of the heat pump cycle such that the flow refrigerant is reversed, and the evaporator becomes the condenser of the system, thereby melting the ice on the exterior surfaces of the outdoor coil. With this method, the heat within the structure being serviced by the heat pump is actually pumped to the outside, thus actually cooling the structure. Accordingly, a backup heat source such as an electric resistive heater must be employed to maintain the temperature within the structure during the defrost operation. Thus, this procedure, like the first defrost procedure, also requires the expenditure of additional energy to compensate for undesirable cooling resulting from the defrost operation.
Attempts have been made to eliminate or at least alleviate some of the disadvantages of traditional defrost procedures. One such procedure is discussed in U.S. Pat. No, 4,420,943, which issued to Lawrence G. Clawson on Dec. 20, 1983. This procedure employs a thermal mass which is located in parallel with a condenser and which receives compressed refrigerant from a compressor. The compressed refrigerant transfers heat to the thermal mass which stores the heat for a subsequent defrost operation. During the defrost operation, the compressor is deactivated and a solenoid valve is opened to fluidly connect the thermal mass to the outlet of the evaporator in bypass of the compressor. With this bypass valve open, the pressures of the evaporator and the condenser equalize to an intermediate pressure. An inventory of refrigerant in contact with the thermal mass boils in the reduced pressure, thereby drawing heat from the thermal mass. The now vaporized refrigerant flows through the bypass valve to the evaporator and condenses in the relatively cool environment, thereby giving off heat to the evaporator which melts ice on the outside of the evaporator.
This defrosting procedure is more energy efficient than other prior art procedures. That is, neither the compressor nor any external heating element need be activated to effect the defrost operation. Moreover, since most of the heat of this defrost system is supplied by the thermal mass, this system does not require the addition of an auxiliary heating device to restore heat removed from the indoor space during the defrost process.
However, this passive defrost system suffers from several disadvantages. First, the thermal mass derives heat from the hot gas leaving the compressor making such heat unavailable for the space heating function. Second, the rapid pressure equalization between the indoor condenser and the outdoor evaporator results in some undesirable heat transfer from the surroundings to the condenser. Moreover, because the thermal mass is located in parallel with the condenser, it does not in any way facilitate cooling of the liquid refrigerant being circulated through the system during the normal thermodynamic cycle taking place while the compressor is operating, and thus does not increase the overall efficiency of the device during normal operation. In addition, the provision for a certain inventory of liquid refrigerant in the thermal mass is difficult to determine because of the variable amount of heat necessary to defrost the evaporator at different conditions. As for example, one pound of refrigerant R-22 will provide only about 70 BTUs of heat as it evaporates from the thermal mass and condenses in the evaporator, such amount is only sufficient to melt about half a pound of ice. Since several pounds of ice can form on the evaporator of a typical residential heat pump, the amount of refrigerant to be inventoried in the thermal mass can become impractically large and in turn create refrigerant charge balancing problems for the heat pump system.